The present invention relates to the method of preparation for a degradable, water and oil resistant protective membrane.
Although the disposable tableware (lunch box, bowl, plate and cup) made mainly from polymeric foam of polystyrene has the advantage of lower price, their applications have been greatly hindered due to the existence of xe2x80x9cdiphenol hormone effectxe2x80x9d and xe2x80x9cwhite polymeric foam pollutionxe2x80x9d. In Chinese patent documents CN 1,255,508A and CN 1,273,255A, it was disclosed that starch was used as the main raw material for the manufacture of disposable tableware and packaging utensils. Although large amount of starch was used and the resources of the starch are abundant, tableware made from them could not withstand water and oil satisfactorily. Chinese patent documents CN 1,234,802A disclosed the use of straw as the main raw material for the manufacture of disposable tableware. The product thus obtained had the color of soil yellow, soil white or soil brown which was unacceptable and in addition an adhesive was needed. In Chinese patent documents CN 1,206,675A, paper pulp was used as the main raw material for the manufacture of disposable tableware. In that case the main raw material was wood pulp and thus serious limitation was imposed on the raw material. Furthermore, in that technique, polyvinylidene chloride blended with polyacrylate should be used as a water-resisting agent.
U.S. Pat. Nos. 3,585,168, 3,900,424 and 3,953,383 disclosed the use of alkyl zinc/active hydrogen containing compound as the catalyst to prepare alternating copolymer (Mn greater than 20,000) of carbon dioxide and epoxide as well as different kinds of polyurethanes and polyethers. Japanese patent kokai publications JP 02,575,199 and JP 02,142,824 disclosed the use of expensive metal coordination compound of porphyrin as the catalyst. But the polymer obtained had relatively low molecular weight (about 5,000). In Chinese patent No. ZL 89100701.6 and ZL 91109459.8, the obtained polymers were difficult to be purified and their molecular weight needed to be raised. Chinese patent document CN 1,257,885 provided a method of preparation for high molecular weight aliphatic polycarbonate. Chinese patent document 00136189.9 disclosed the use of rare earth catalyst to synthesize a copolymer of epoxide and carbon dioxide with a molecular weight of 80,000-200,000, a content of alternating structure larger than 98% and a degree of fixation of carbon dioxide over 40 wt %. The said copolymer of carbon dioxide is soluble in solvents such as dichloromethane, chloroform, tetrahydrofuran, dioxane, butyl formate, ethyl acetate, butyl acetate etc.
Copolymers of carbon dioxide, aliphatic polycarbonate with high content of alternating structure synthesized from carbon dioxide and epoxide are not only photodegradable but also biodegradable plastics due to existence of ester bonds in its main chain. Carbon dioxide copolymer membrane with high molecular weight possesses good transparency and good barrier property to oxygen and water. They are expected to have wide applications in the fields of disposable tableware, degradable packaging material for pharmaceutics and foodstuff as well as composites.
The objective of the present invention is to provide a method of preparation for a degradable, water and oil resistant protective membrane or coating. The said protective membrane mainly consists of a copolymer of carbon dioxide and epoxide. It possesses good oxygen barrier property. The said carbon dioxide copolymer is dissolved in a low toxic or non-toxic solvent to form a solution of concentration of 3-6 wt %. The surface of the fabricated starch tableware is then coated with this solution by spraying or dipping. The residual solvent is removed by drying process and a degradable, water and oil resistant and transparent protective membrane is obtained.
The binary copolymers of epoxide and carbon dioxide used in the present invention are those of ethylene oxide-carbon dioxide; propylene oxide-carbon dioxide, styrene oxide-carbon dioxide and cyclohexene oxide-carbon dioxide. The molecular weights of the binary copolymer are in the range of 80,000-200,000. Their contents of alternating structure are large than 98% with carbon dioxide content over 40 wt %. The ternary copolymers used are those of epoxide-carbon dioxide-acid anhydride. The epoxides used are ethylene oxide, propylene oxide, styrene oxide or cyclohexene oxide. The acid anhydrides used are maleic anhydride, fumaric anhydride or succinic anhydride. The molecular weights of the ternary copolymers are 80,000-200,000. Their contents of alternating structure are larger than 98% with carbon dioxide content over 40 wt %.
According to Chinese patent CN 1,257,885, a typical procedure to prepare the said carbon dioxide copolymer was described as follows. The aged Nd(CCl3COO)3/ZnEt2/glycerin(containing 0.00075 mol, 0.015 mol ZnEt2, 0.015 mol glycerin, and 0.150 mol propylene carbonate, aged at 20 atm of CO2 atmospheric pressure for 12 hrs) was used as the catalyst. The catalyst and 80 ml propylene oxide were put into the autoclave in the absence of oxygen, and then CO2 was quickly filled in till 30 atm. The polymerization was carried out at 70xc2x0 C. for 10 hrs. The reaction was terminated by addition of 5% HCl/methanol solution, and the precipitate was dried under vacuum till constant weight. The propylene oxide conversion was 69%. The final yield was 8.04xc3x97104 g polymer/mol Nd. 1H NMR measurement showed that the content of alternative sequence structure was over 98%. Number average molecular weight was 140,000, and the glass transition temperature was 39.1xc2x0 C.
A typical procedure for preparing the ternary copolymer of present invention is as follows. The aged Nd(CCl3COO)3/ZnEt2/glycerin (containing 0.00075 mol, 0.015 mol ZnEt2, 0.015 mol glycerin, and 0.150 mol propylene carbonate, aged at 20 atm of CO2 atmospheric pressure for 12 hrs) was used as the catalyst. The catalyst and 75 ml propylene oxide and 5 ml of cyclohexene oxide were put into the autoclave in the absence of oxygen, and then CO2 was quickly filled in till 40 atm. The copolymerization was carried out at 70xc2x0 C. for 10 hrs. The reaction was terminated by addition of 5% HCI/methanol solution, and the precipitate was dried under vacuum till constant weight. The final yield was 7.1xc3x97104 g polymer/mol Nd. Number average molecular weight was 110,000, and the glass transition temperature was 60.5xc2x0 C.
Solvents used in this invention include one of the following solvents: dichloromethane, chloroform, tetrahydrofuran, dioxane, butyl formate, ethyl acetate, propyl acetate, ethyl propionate, methyl butyrate, butyl acetate or mixed solvent of more than two of the above-mentioned solvents. The preferred solvent is one or more of esters selected from the group consisting of butyl formate, ethyl acetate, propyl acetate, ethyl propionate, methyl butyrate and butyl acetate. The ratios of the mixed solvents are 95-98:5-2; 95-98:3-1:2-1. The embodiment of the process is as follows: The binary or ternary copolymer of epoxide and carbon dioxide is dissolved in a low toxic or non-toxic solvent to form a solution of concentration of 3-6 wt %. The solution is allowed to stay for 2-12 hours and then filtered or centrifuged. The filtrate is allowed to stay for another 2-6 hours and followed by filtration or centrifugation to remove the residual metal oxide. The content of metal ions in the filtrate should be below 5.0 ppm. The said filtrate of binary or ternary copolymer solution of carbon dioxide with metal ion content lower than 5.0 ppm is used directly to coat the surface of tableware made of starch, straw or paper pulp by spraying or dipping. The thickness of the protective membrane on the surface of starch, straw or paper pulp vessel is in the range of 10-20 micrometer. The weight of copolymer is 8-12 wt % of that of tableware. Metal ion content in the tableware is lower than 1 ppm. No leakage and deformation of the tableware are observed after the tableware is immerged in water or edible oil for 4 hours at 80xc2x0 C. The decoloration test by cold edible oil and ethanol is negative. Fungus test gives grade V.